With the rise of the environmental protection movement in recent years, emission regulations for carbon dioxide and similar gases causing global warming have been made more stringent and the automobile industry is vigorously developing electric vehicles (EVs) and hybrid electric vehicles (HEVs), in addition to automobiles using fossil fuels such as gasoline, diesel oil, and natural gas. Nickel hydrogen secondary batteries or lithium ion secondary batteries have been used as the batteries for these EVs and HEVs. In recent years, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries, which are lightweight and have high capacity, have come into common use.
Batteries for EVs and HEVs are also required to achieve, in addition to environmental responsiveness, a highly developed traveling capacity in a basic performance of automobiles, such as an acceleration performance or a hill climbing performance. In order to achieve these requirements, it is not only necessary to increase battery capacity, but also increase the battery output power. Generally, prismatic sealed batteries having a structure in which electric power generating elements are accommodated in a prismatic outer can made of an aluminum-based metal are used as nonaqueous electrolyte secondary batteries for EVs and HEVs. When a battery is discharged at high power, a high current is applied to the battery. Therefore, the battery needs to have low resistivity and the internal resistance needs to be reduced as much as possible. Also, various improvements have been made to achieve low resistivity in terminal portions.
A mechanical crimping method has been used most commonly as a method for realizing low resistivity in terminal portions of these batteries. However, performing only mechanical crimping will cause chronological change in electrical resistivity under conditions in which vibration is frequently applied such as in EVs and HEVs, thus the mechanical crimping method is used in combination with a laser welding method. An example of electrically joining a collector and a terminal by combining the mechanical crimping method and the laser welding method disclosed in JP-A-2004-14173 will be explained with reference to FIGS. 6A and 6B. FIG. 6A is a sectional view showing, in an upside down manner, a joint section of a collector and a terminal portion disclosed in JP-A-2004-14173, and FIG. 6B is a sectional view of the joint section of the collector and the terminal of FIG. 6A before laser welding.
A joint section 50 of the collector and the terminal disclosed in JP-A-2004-14173 includes, a cover plate 51 fixed to a battery outer body (not shown), an internal insulating sealing member 52 and an outer insulating sealing member 53, a collector 54 coupled to electric power generating elements, and a rivet terminal 55, as shown in FIG. 6A. The internal insulating sealing member 52 and the outer insulating sealing member 53 include through holes and are arranged at the edges of both inner and outer circumferences of an opening formed in the cover plate 51. The collector 54 includes a terminal hole and a projecting body 54a hanging down along the terminal hole, and is arranged over the internal insulating sealing member 52. The rivet terminal 55 includes a rivet portion 55b projected from and connected to a jaw portion 55a. 
The joint section 50 is assembled as follows. The rivet portion 55b of the rivet terminal 55 is inserted through, from the outer circumference side of the cover plate 51, the through holes of the internal insulating sealing member 52 and the outer insulating sealing member 53, the opening of the cover plate 51, and a rivet terminal hole of the collector 54. Next, the rivet portion 55b of the rivet terminal 55 is crimped so as to press the projecting body 54a of the collector 54, and thereafter, a welded portion 56 is form by laser welding the rivet portion 55b and the collector 54.
By the crimping method of the related-art example shown in FIG. 6A, as shown in FIG. 6B, a tip end 55c of the crimping member after crimping the rivet portion 55b of the rivet terminal 55 is thinner towards the tip because the rivet portion 55b is thinned and extended towards the tip. Thus, a gap 57 is formed between the projecting body 54a of the collector 54 and the tip end 55c of the crimping member. Therefore, welding of the projecting body 54a of the collector 54 and the tip end 55c of the crimping member is performed by obliquely irradiating the gap with a laser beam at about 45 degrees. The laser beam is multiply reflected within the gap so as to fuse and join the tip end 55c of the crimping member and the projecting body 54a of the collector 54. Thus, the welded portion 56 as shown in FIG. 6A is obtained.
Assist gas is used for such a laser welding. Since the assist gas does not thoroughly enter the periphery of the welding portion 56 within the gap, metal fine powder sputtered during laser irradiation is oxidized, whereby soot-like metal oxidized fine particles are adhered on the welding member. In addition, when industrially manufacturing the batteries, laser welding of the terminal portions is performed in the assembly line, and therefore, deviation of laser irradiation position with respect to the welding portion 56 of the batteries may occur. When the laser beam irradiation position deviates to the projecting body 54a side of the collector 54, the amount of laser beam to be irradiated on the tip end 55c side of the crimping member of the rivet terminal 55 decreases, and therefore, the welding becomes insufficient. On the other hand, when the laser beam irradiation position deviates to the tip end 55c side of the crimping member of the rivet terminal 55, the surface on the tip end 55c side of the crimping member 55b is sputtered, and therefore, normal laser welding cannot be performed. In addition, in the above related-art example, since the laser is irradiated from an oblique direction with respect to the welding member, the welding member needs to be rotated in order to irradiate multiple portions symmetrically. Therefore, the manufacturing device becomes complicated.
In order to solve the issues of the above related-art example, JP-A-2008-251411 discloses the following example. A processing punch having a specific shape is used to form the tip end side of a crimping portion of a terminal so as no gaps are formed between a collector and the tip end of the crimping portion. A laser beam is irradiated with respect to the tip end of the crimping portion, and the terminal and collector are laser welded. The laser welding method of the collector and crimping portion disclosed in JP-A-2008-251411 will be explained with reference to FIGS. 7A and 7B. FIG. 7A is a sectional view showing a processing step of the tip end of the crimping portion of the terminal disclosed in JP-A-2008-251411, and FIG. 7B is a diagram showing a step of laser welding after the step of FIG. 7A.
A joint section 60 of the collector and the terminal disclosed in JP-A-2008-251411 includes a cover plate 61 fixed to a battery outer body (not shown), an internal insulating sealing member 62 and an outer insulating sealing member 63, a collector 64 coupled to electric power generating elements, and a rivet terminal 65. The internal insulating sealing member 62 and the outer insulating sealing member 63 include through holes and are arranged at the edges of both inner and outer circumferences of an opening formed in the cover plate 61. The collector 64 is arranged over the internal insulating sealing member 62. The rivet terminal 65 includes a crimping portion 65b projected from and coupled to and a jaw portion 65a. 
The joint section 60 is assembled as follows. The crimping portion 65b of the rivet terminal 65 is inserted through, from the outer circumference side of the cover plate 61, the outer insulating sealing member 63, the opening of the cover plate 61, the internal insulating sealing member 62, and a rivet terminal hole of the collector 64, and are integrated by crimping the crimping portion 65b of the rivet terminal 65 by pressing the collector 64. Steps up to here are substantially the same as those disclosed in JP-A-2004-14173 as shown in FIGS. 6A and 6B. A processing punch A, which includes a recessed portion complementary to the crimping portion 65b of the rivet terminal 65, and also includes an inclined portion A1 having a predetermined angle with respect to the peripheral edge of the recessed portion, is prepared. The processing punch A is pressed in so as the inclined portion A1 abuts the tip end 65a of the crimping portion 65b. The tip end 65c of the crimping portion 65b is partially deformed, and the tip end 65c of the crimping portion 65b is formed into a circular truncated cone portion, as shown in FIG. 7B. Thus, the shape of the tip end 65c of the crimping portion 65b is adjusted to have a blunt angle.
Next, the laser welding is performed by irradiating the top surface of the circular truncated cone portion of the tip end 65c of the crimping portion 65b with the laser beam LB in the perpendicular direction or in the direction close to the perpendicular direction. At this time, the irradiation range of the laser beam LB is the area including at least the collector 64 and the circular truncated cone portion of the tip end 65c of the crimping portion 65b. With this laser welding, energy of the laser irradiated to both of the collector 64 and the circular truncated cone portion of the tip end 65c of the crimping portion 65b is transferred without being distortion, whereby a favorable welding spot (nugget) 66 is formed in the welding portion.
Furthermore, in order to solve the problems of the above related-art techniques, JP-A-2009-087693 discloses an example of laser welding a terminal and collector by forming a tip end side of a crimping portion of a rivet terminal into a thin-walled portion by using a processing punch having a specific shape, by leaving no gaps between the collector and the tip end of the crimping portion, and by irradiating a laser beam with respect to the thin-walled portion of the crimping portion. The laser welding method of the collector and crimping portion disclosed in JP-A-2009-087693 will be explained with reference to FIGS. 8A and 8B. FIG. 8A is a sectional view showing the state after the crimping step of the rivet terminal disclosed in JP-A-2009-087693, and FIG. 8B is a diagram showing the step of laser welding after the step in FIG. 8A.
A joint section 70 of a collector and a terminal disclosed in JP-A-2009-087693 also includes a cover plate 71 fixed to a battery outer can (not shown), an internal insulating sealing member 72 and an outer insulating sealing member 73, a collector 74 coupled to electric power generating elements, and a rivet terminal 75. The internal insulating sealing member 72 and the outer insulating sealing member 73 include through holes, and are arranged at the edges of both inner and outer circumferences of an opening formed in the cover plate 71. The collector 74 is arranged over the internal insulating sealing member 72. The rivet terminal 75 includes a crimping member 75b projected from and connected to a jaw portion 75a. 
The joint section 70 is assembled as follows. The crimping portion 75b of the rivet terminal 75 is inserted through, from the outer circumference side of the cover plate 71, the through hole of the outer insulating sealing member 73, the opening of the cover plate 71, the internal insulating sealing member 72, and a rivet terminal hole of the collector 74. Next, the crimping portion 75b of the rivet terminal 75 is integrated by crimping by pressing the collector 74. At this time, the crimping portion 75b of the rivet terminal 75 is formed with a thin-walled portion 75d having a small thickness annularly by using a punch having its peripheral portion annularly projected as a crimping jig. The thin-walled portion 75d of the crimping portion 75b of the rivet terminal 75 thoroughly adheres the collector 74, and also a portion with a flat surface is formed annularly.
Next, the thin-walled portion 75d of the crimping portion 75b is laser welded by irradiating the laser beam LB in the perpendicular direction or in a nearly perpendicular direction. At this time, the irradiation range of the laser beam LB may be anywhere within the thin-walled portion 75d of the crimping portion 75b of the rivet terminal 75. With this laser welding, the laser is surely irradiated to the surface of the thin-walled portion 75d of the crimping portion 75b even if the laser beam irradiation position deviates, whereby a favorable nugget 76 is formed in the welding portion.
The structure of the joint section 60 disclosed in JP-A-2008-251411 has an advantage that the welding portion does not have to be rotated even when symmetrically laser welding multiple portions since the laser beam can be irradiated from above the welding portion. However, this structure also presents an issue. While the area including the collector 64 and the circular truncated cone portion of the tip end 65c of the crimping portion 65b is being welded, preferable welding cannot be performed if the laser irradiation position deviates to the crimping portion side due to the large thickness of the crimping portion 65b. In addition, if the laser irradiation position deviates to the collector 64 side, the welding portion becomes small, and the strength of the welding portion becomes weak.
The structure of the joint section 70 disclosed in JP-A-2009-087693 has the same advantage as those of the joint section 60 disclosed in JP-A-2008-251411, and also has an advantage that the terminal portion and the collector can be favorably laser welded even if the laser irradiation position deviates due to the small thickness of the crimping portion 75b. However, the joint section 70 disclosed in JP-A-2009-087693 has the collector 74 melted after the melt portion of the crimping portion 75b penetrates the crimping portion 75b by the laser beam irradiation, and therefore, there is a problem that the laser beam output has to be large, and also the laser beam irradiation time has to be extended in order to increase the welding strength.